1. Field of the Invention
This invention relates to electric circuits, and more particularly to a circuit adapted to be implemented in integrated circuit form as an auxiliary to a digital to analog converter.
2. Description of the Prior Art
Digital to analog converters (DACs) employ a plurality of stages or bits to convert a multi-signal digital input to a unitary signal analog output. Assuming a binary coded input, one bit would be assigned to each digit of the binary code. For a given binary input, some of the digits would carry a logic "one", and the other digits a "zero". Those bits associated with the digits having a "one" each produce an output signal, the magnitude of which is proportional to the order of the digit. The individual bit outputs are accumulated to produce a net output which is the analog equivalent of the digital input signal.
In operating a DAC, the ability to discriminate between a "one" and a "zero" is essential. A common method for accomplishing this employs a differential switch in each bit. One branch of the switch is biased at a constant voltage which is less than the logic input signals but great enough to exclude the effects of normally encountered noise, while the second branch is biased by the logic signal itself. During the presence of a logic "one" at the second branch, the bit current is directed through the switch to an output terminal which also receives current from all of the other bits to which a logic "one" has been applied. Those bits which have received logic inputs exceeding the threshold established by the bit bias level contribute to the total output analog signal, while the remaining bits do not.
Numerous different logic systems have been developed, each having its own particular characteristics, advantages, and disadvantages. Each logic system with which the DAC may be used has associated with it a threshold bias level for switching the bits. In several logic systems the bit threshold level is relatively low. Such systems include positive doped metal oxide semiconductor (PMOS) logic, negative doped metal oxide semiconductor (NMOS) logic, emitter coupled logic (ECL), and integrated injection logic I.sup.2 L. In all of these systems, the threshold bias level for bit switching is less than 1.4 volts, and generally about zero. For CMOS, unlike the first category of logic systems, the bit threshold level depends upon the supply voltage and is generally equal to one half of the positive voltage supply. A third category comprises transistor transistor logic (T.sup.2 L), in which the bit threshold level is about 1.4 volts.
In view of the disparate types of logic systems available a complete replacement of a DAC may be necessary to convert from operation in one category to operation in another. Greater economy and efficiency can be achieved by providing a universal DAC capable of being programmed for compatibility with any of the above logic systems. Different DAC manufacturers have recently recognized this fact and developed means of adjusting the bit switching threshold with a programmable or user selected logic control voltage. However, up to this point there does not appear to be any uniformity in the selection of the relationship between the bit switching threshold and the logic control voltage. For example, some manufacturers require that the logic control voltage be forced to zero by grounding in order to achieve a switching threshold at 1.4 volts, while others require that the logic control input be left open circuited to achieve the 1.4 volt threshold. Thus, different DACs designed for a particular logic system may require different control. In addition, most manufacturers provide programmability for only two of the three general categories of logic systems discussed above, since providing for all three simultaneously produces some design conflicts.